In such an air classifier, the static sifter has a plurality of impact and conducting elements stepped one below the other inside a housing having at least one first material inlet, at least one sifting-gas inlet, and at least one coarse-particle outlet, and the dynamic sifter is a rod-basket sifter with a rotary rod basket and having a second housing with at least one medium-particle outlet and one fine-particle outlet.
The granular material to be sifted can be, for example, cement, cement-containing materials, cement raw material, limestone or slag, but also ore and the like. Roll crushers and/or material bed roll mills are used in practice for the comminution of such granular materials. In this high-pressure comminution of granular material, such granular material is crushed in the nip between two press rollers (material bed comminution). Agglomerates are formed during the comminution action, which are referred to as scales. Such material bed roll mills can be operated in a closed circuit with a static and/or dynamic sifter. The material bed roll mill is positioned therein, for example, below a sifter such that any coarse material fraction that is discharged from the sifter is fed back to the roll mill. The material discharged from the roll mill is refed to the material inlet of the air classifier that is a multistage apparatus including a static sifter and a dynamic sifter. The scales are deagglomerated in the static sifter by the impact and conducting elements; simultaneously, the coarse material fraction is separated and fed to the roll crusher. The “finer” material arrives together with the sifting gases in the dynamic sifter where it is subjected to a fine-sifting process. The fine material that is sifted from this sifter is discharged together with the sifting gas and captured as finished material in the cyclones and/or filters that follow downstream. The medium fraction that is sifted from the dynamic sifter can also be refed, for example, to the roll crusher or a further milling stage. Operative measures of this kind of are known from the prior art (see, for example, DE 43 37 215 [U.S. Pat. No. 5,505,389].
An air classifier according to the type descried above is disclosed, for example, in DE 42 23 762. It has a rotary powered rod basket that is provided in a housing with turbo elements angularly arrayed around the rotor, with inlets and outlets for sifting air, material to be sifted, fine material, medium material and coarse material. A chute-like presifting chamber extends horizontally upstream and to the side of the rod basket at the same height and has, at the top, an inlet opening for the material to be sifted that is separated from the sifting air and an opening for the sifting air that is laterally opposite the rod basket, and, at the bottom, an outlet opening for a sifted coarse-grained fraction and two chute-delimiting walls that are permeable for the sifting air and that together form a presifting zone. These chute-delimiting walls of the presifting chamber that are permeable for the sifting air and include angled blind-like metal guides that are tilted toward the bottom toward the outlet opening of the sifted coarse material fraction and provide, as impact and conducting elements, deagglomeration action for the scale.
It was further proposed to provide roof-shaped assemblies that are arranged in a cascade on air classifiers, such that the edge of the ridge of each assembly is approximately perpendicular below the discharge edge of the surface of the assembly, which is provided upstream (see DE 1 002 600).
WO 2003/097241 [U.S. Pat. No. 7,300,007] discloses a further air classifier of this type where the dynamic sifter is equipped with a rod basket that rotates about a horizontal axis, as in DE 42 23 762. To minimize problems involved in the mechanical conveying of the grinding stock that is transported in the circuit, this known prior art proposes mounting the static cascade sifter below the roll nip of the roll crusher, and the post-sifter above this roll crusher that is designed, in particular, as a dynamic rod-basket sifter. Disadvantageously, however, this embodiment has a considerable structural height. The connecting line between the two sifters increases the investment and operating costs.
An alternate embodiment of an air classifier in multiple stages and compact structural form is disclosed in U.S. Pat. No. 7,854,406. The air classifier therein has a plurality of concentric housings, with a rotating rod basket that rotates about a vertical axis as a post-sifting stage. The presifting stage is constituted by a simple cyclone, and the material to be sifted and the sifting gas are fed via a common supply line that is connected to the housing in a spiral. Any deagglomeration of the scale is only possible to a limited degree in the course of the static sifting stage.
DE 10 2004 027 128 discloses an air classifier for granular material for obtaining at least three grain fractions and having a static sifter and a dynamic sifter in a rotationally symmetrical array about a common axis inside a common housing.
Finally, DE 10 2006 039 775 discloses an air classifier in a special modality of construction having a static cascade sifter and a further sifter as a post-sifter, wherein the cascade sifter has two packs of conical rings that are provided spaced one above the another and concentrically relative to one another.
DD 253771 discloses, furthermore, a air classifier for separating, in particular, fine-grained bulk materials into at least two fractions having a cylindrical top housing part extended downward as a granule-type cone with granule discharge. The rod basket therein rotates about a vertical axis. The material distribution inside the sifter chamber of sifters with rod baskets is to be improved in that the precision at separation is increased and energy consumption with regard to the final product is reduced, irrespective of the speed and form of a given rotor. To this end, an annular container with rotating base is provided as a dispersing apparatus that is above the sifting-gas inlet pipe in the area of the rod basket inside or outside of the housing and connected to the sifter chamber via an annular gap and/or an annular channel.
The known sifters of the type as previously described have basically proven themselves useful in practice; however, there still remains the potential for improvement, particularly with regard to the sifter efficiency.